1. Technical Field of the Invention
The present invention relates to an optical head apparatus and an optical information recording and playback apparatus which record on and play back from an optical recording medium, and particularly relates to an optical head apparatus and an optical information recording/playback apparatus which are capable of detecting a radial tilt of an optical recording medium.
2. Description of the Prior Art
A recording density in an optical information recording/playback apparatus is inversely proportional to a square of a diameter of a focused spot formed on an optical recording medium by an optical head apparatus. As the diameter of the focused spot becomes smaller, the recording density becomes higher. The diameter of the focused spot is inversely proportional to a numerical aperture of an objective lens in the optical head apparatus. Further, as the numerical aperture of the objective lens becomes higher, the diameter of the focused spot becomes smaller. Meanwhile, when the optical recording medium is tilted to a radial direction with respect to the objective lens, a shape of the focused spot is distorted due to a coma aberration caused on a substrate of the optical recording medium, and recording/playback characteristics are deteriorated. Since the coma aberration is proportional to a cube of the numerical aperture of the objective lens, as the numerical aperture of the objective lens becomes higher, a margin of a tilt of the optical recording medium in the radial direction with respect to the recording/playback characteristics (radial tilt) becomes smaller. Therefore, in the optical head apparatus and the optical information recording/playback apparatus in which the numerical aperture of the objective lens is made high in order to increase the recording density, it is necessary to detect and compensate the radial tilt of the optical recording medium so that the recording/playback characteristics are not deteriorated.
FIG. 27 shows a structure of a conventional optical head apparatus which is capable of detecting the radial tilt of the optical recording medium. This optical head apparatus is described in Japanese Patent Application Laid-Open No. 7-141673 (1995). A beam emitted from a semiconductor laser 257 is converted into a parallel beam by a collimating lens 258, and about 50% of the beam is transmitted through a half mirror 259 and is focused on a disc 261 by an objective lens 260. A beam reflected by the disc 261 is transmitted through the objective lens 260 in the opposite direction, and about 50% is reflected by a half mirror 259, and is divided into a transmitted beam and diffracted beams by a holographic element 262. The beams are transmitted through a lens 263 and are detected by a photo detector 264.
FIG. 28 is a plan view of the holographic element 262. The holographic element 262 has elliptical grating regions 265 and 266 which are positioned on the radial direction of the disc 261. The directions of the gratings in both the regions 265 and 266 are approximately parallel with a tangential direction of the disc 261, and the pattern of the gratings in both the regions 265 and 266 is off-axis concentric shape. Beams incident to the regions 265 and 266 are partially or fully diffracted as +1st order beams. Meanwhile, beams incident to the outside of the regions 265 and 266 are fully transmitted. Here, a dotted line in FIG. 28 shows an effective diameter of the objective lens 260.
FIG. 29 shows a pattern of detection portions of the photo detector 264 and an arrangement of focused spots on the photo detector 264. A focused spot 271 corresponds to a beam transmitted from the outside of the regions 265 and 266 of the holographic element 262, and it is received by detection portions 267 and 268 which are divided into two by a dividing line passing through an optical axis and parallel with the tangential direction of the disc 261. A focused spot 272 corresponds to the +1st order beam diffracted by the inside of the region 265 of the holographic element 262, and it is received by a single receiving area 269. A focused spot 273 corresponds to the +1st order beam diffracted by the inside of the region 266 of the holographic element 262, and it is received by a single receiving area 270.
When outputs from the detection portions 267 to 270 are represented by V267 to V270 respectively, a tracking error signal is obtained by calculation of (V267+V269)xe2x88x92(V268+V270) according to the push-pull method. A radial tilt signal for detecting a radial tilt of the disc 261 is obtained by calculation of (V267+V270)xe2x88x92(V268+V269). Moreover, a playback signal is obtained by calculation of (V267+V268+V269+V270). A method of obtaining a focusing error signal is not described.
There is explained below the reason the radial tilt of the disc 261 can be detected by the above-mentioned calculation with reference to FIGS. 30 to 32. FIGS. 30 to 32 show calculation examples of intensity distribution of the reflected beam from the disc 261. In the drawings, a dark portion corresponds to a portion where the intensity is strong, and a beaming portion corresponds to a portion where the intensity is weak.
FIG. 30 shows the intensity distribution in the case where the disc 261 does not have the radial tilt. The intensity distribution is symmetrical with respect to a straight line which passes through the optical axis and is parallel to the tangential direction of the disc 261. Further, the intensity is comparatively strong in regions 274 and 276 where the 0th order beam overlaps with the +1st order beam diffracted by the disc 261. The intensity is also comparatively strong in regions 275 and 277 where the 0th order beam overlaps with the xe2x88x921st order beam diffracted by the disc 261. On the contrary, the intensity is comparatively weak in a region 278 where there is only the 0th order beam from the disc 261.
FIG. 31 shows the intensity distribution in the case where the disc 261 has a positive radial tilt. As for regions 279 and 281 which are regions where the 0th order beam and the +1st order beam diffracted by the disc 261 are overlapped with each other, the intensity in the region 279 as a peripheral area is stronger than the intensity in the region 281 as a central section. As for regions 280 and 282 which are regions where the 0th order beam and the xe2x88x921st order beam diffracted by he disc 261 are overlapped with each other, the intensity in the region 280 as a peripheral area is weaker than the intensity in the region 282 as a central section.
FIG. 32 shows the intensity distribution in the case where the disc 261 has a negative radial tilt. As for regions 283 and 285 which are regions where the 0th order beam and the +1st order beam diffracted by the disc 261 are overlapped with each other, the intensity in the region 283 as a peripheral area is weaker than the intensity in the region 285 as a central section. As for regions 284 and 286 which are regions where the 0th order beam and the xe2x88x921st order beam diffracted by the disc 261 are overlapped with each other, the intensity in the region 284 as a peripheral area is stronger than the intensity in the region 286 as a central section.
In FIGS. 30 to 32, the peripheral area and the central area in the region where the 0th order beam and the +1st order beam diffracted by the disc 261 are overlapped with each other correspond to the detection portions 267 and 269 of the photo detector 264 shown in FIG. 29, and the peripheral area and the central area in the region where the 0th order beam and the xe2x88x921st order beam diffracted by the disc 261 are overlapped with each other correspond to the detection portions 268 and 270 of the photo detector 264 shown in FIG. 29.
When the radial tilt of the disc 261 is zero, positive and negative, a value of (V267+V270)xe2x88x92(V268+V269) as the radial tilt signal becomes zero, positive and negative respectively. Therefore, this radial tilt signal is used to detect the radial tilt of the disc 261. When the radial tilt of the disc 261 is detected, the radial tilt is corrected so as to eliminate a bad influence on the recording/playback characteristics.
In the conventional optical head apparatus, when the objective lens 260 shifts to the radial direction of the disc 261 due to eccentricity or the like of the disc 261, an offset is generated in the radial tilt signal. Therefore, the radial tilt of the disc 261 cannot be correctly detected by the conventional optical heads. Concretely, when the objective lens 260 shifts to the radial direction of the disc 261, the focused spot 271 on the photo detector 264 also shifts to the radial direction of the disc 261. In the case where the focused spot 271 shifts to the left in FIG. 29, an output from the beam receiving area 267 increases and an output from the beam receiving area 268 decreases. For this reason, a positive offset is generated in [(V267+V270)xe2x88x92(V268+V269)] as the radial tilt signal.
It is an object of the present invention to provide an optical head apparatus and an optical information recording/playback apparatus, wherein even if an objective lens shifts to the radial direction of an optical recording medium, an offset is not generated in a radial tilt signal, and a radial tilt of the optical recording medium can be detected correctly.
In an optical head apparatus of the present invention, a main beam and a sub beam are generated from a beam emitted from a beam source. The main beam and the sub beam reflected by an optical recording medium are divided into four regions, namely, R1 (a peripheral area of a region where a 0th order beam and a +1st order beam diffracted by the optical recording medium are overlapped with each other), R2 (a central area of a region where the 0th order beam and the +1st order beam diffracted by the optical recording medium are overlapped with each other), R3 (a peripheral area of a region where the 0th order beam and a xe2x88x921st order beam diffracted by the optical recording medium are overlapped with each other), and R4 (a central area of a region where the 0th order beam and the xe2x88x921st order beam diffracted by the optical recording medium are overlapped with each other). The radial tilt signal is proportional to the total intensity of R1 and R4 minus the total intensity of R2 and R3. In order to obtain the radial tilt signal, the sub beam is arranged to be shifted in a radial direction of the optical recording medium with respect to the mainbeam.
Here, let""s compare the component of the total intensity of R1 and R4 minus the total intensity of R2 and R3 contributed by the main beam (radial tilt signal by main beam) with the component of the total intensity of R1 and R4 minus the total intensity of R2 and R3 contributed by the sub beam (radial tilt signal by sub beam).
The radial tilt signal by main beam is different from the radial tilt signal by sub beam, when the optical recording medium has a radial tilt. Meanwhile, in the case where the objective lens is shifted in the radial direction of the optical recording medium, since the shift of the main beam on the photo detector is the same as that of the sub beam, an offset generated in the radial tilt signal by main beam is the same as that generated in the radial tilt signal by sub beam.
Therefore, when a difference between the radial tilt signal by main beam and the radial tilt signal by sub beam is a final radial tilt signal, in the case where the optical recording medium has a radial tilt, the radial tilt signals by main beam and by sub beam are not canceled each other, and offsets generated in the radial tilt signals by main beam and by sub beam in the case where the objective lens is shifted in the radial direction of the optical recording medium are canceled each other. For this reason, even if the objective lens is shifted in the radial direction of the optical recording medium, an offset is not generated in the radial tilt signal, and the radial tilt of the optical recording medium can be detected correctly.
In an optical information recording/playback apparatus of the present invention, an optical head apparatus of the present invention which is capable of detecting the radial tilt of the optical recording medium is used so that the radial tilt of the. optical recording medium is corrected in order to eliminate a bad influence upon the recording/playback characteristics. According to the present invention, even if the objective lens is shifted in the radial direction of the optical recording medium, an offset is not generated in the radial tilt signal and the radial tilt of the optical recording medium can be detected correctly.